1. Field of the Invention
The present invention relates to a solid-state imaging device having a photodiode and a circuit element such as a MOS transistor that is adjacent to the photodiode, and relates to a method of producing the solid-state imaging device.
2. Description of Related Art
In conventional solid-state imaging devices such as a CMOS image sensor and a CCD image sensor, a sidewall forming film formed of silicon dioxide film (a deposited film used for forming a side wall) and an interlayer insulation film were provided over the light-receiving surface of a photodiode that had been formed on a silicon substrate.
In this case, about 30% of the incident light to the photodiode is reflected by the interface between the silicon substrate and the silicon dioxide film, which prevents the portion of the incident light from contributing to the photoelectric conversion, because the refractive indices of the silicon substrate and the silicon dioxide film are approximately 3.5 and 1.46, respectively.
For this reason, for instance, Japanese Patent Publication JP-A 62-143457 (FIG. 3) discloses a solid-state imaging device in which the loss of the incident light to its photodiode was reduced by providing a silicon nitride film over the light-receiving surface of the photodiode, the nitride film having a refractive index of about 2.1 that is an intermediate value between the silicon substrate and the silicon dioxide film.
However, when the silicon nitride film remains on the device in contact with its circuit elements such as MOS transistors, resistors, and capacitors, the film stress of the silicon nitride film produces cracks in the circuit elements by temperature variation during producing the solid-state imaging device or by secular change in the solid-state imaging device. Thus, there has been the problem that there occurs malfunction of circuit elements, thereby deteriorating the reliability of the solid-state imaging device.
In addition, when the silicon nitride film remains on the photodiode in contact with a device isolation such as LOCOS isolation trench isolation, and pn junction isolation, crystal defects located in the vicinity of the film increase in number by temperature variations during the production process of the solid-state imaging device or by secular changes in the solid-state imaging device. Consequently, there has been the problem that junction leakage currents increase, which may deteriorate the quality of an image picked up in a dark state and stored in the solid-state imaging device.
Moreover, in the case where the silicon nitride film is selectively removed by means of wet etching process by use of hot phosphoric acid, the light-receiving surface of the photodiode is damaged by the heat phosphoric acid. As a result, there has been the problem that it is impossible to selectively remove the silicon nitride film without damaging the light-receiving surface of the photodiode.